Frame-shifting PCR for germline immunoglobulin genes retrieval and antibody engineering

ABSTRACT

A method for preparing an antigen-specific antibody by constructing a library of phage-displayed single chain variable fragment of an antibody with a novel frame-shifting PCR is disclosed. Also disclosed is a method for preparing a clone for producing an antigen-specific antibody.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing an antibody, particularly to a method for preparing an antigen-specific antibody by constructing a library of phage-displayed single chain variable fragment (ScFv) of an antibody using a novel frame-shifting PCR step.

2. Description of the Related Art

Monoclonal antibody (mAb) is mainly derived by cell fusion as described originally by Kohler and Milstein (1975). Owing to the broad application and wide perspective of the monoclonal antibody, important advances in design, selection, and production of engineered antibodies have been made. The traditional method for recombinant mAb construction like hybridoma technology has many limitations, such as duration, stability and class manipulation (Harlow and Lane, (1988) Monoclonal antibody: A laboratory manual pp. 141-149). Although new technology like the display of antibody fragments on the surface of filamentous phages and the subsequent selection of antibodies have been proved as an effective tool for the isolation of antigen specific antibodies (Barbas, C. F., Kang, A. S., Lerner, R. A. & Benkovic, S. J. (1991), Assembly of combinatorial antibody libraries on phage surfaces: the gene III site, Proc. Natl. Acad. Sci., USA 88, 7978-7982; Marks, J. D., (1992), By-passing immunization: building high affinity human antibodies by chain shuffling, Bio/Technology, 10, 779-783; Nissim, A. (1994), Antibody fragments from a ‘single pot’ phage display library as immunochemical reagents, EMBO J. 13, 692-698), it still has limitations. The library of phage-displayed single chain variable fragment (ScFv) of the antibody has been used for deriving tailor-made antigen-specific monoclonal antibody in the last decade. Furthermore, affinity enhancement of ScFv can be achieved by in vitro mutation.

In general, the affinity of isolated antibodies is proportional to the initial size of the library used for selection. Using mRNA as an enriched source of expressed and spliced antibody genes neglects the allelic exclusive genes so as half of the potential genes. Hence, the diversity of the library used for selection becomes limited. Moreover, the gene loss will happen in both self-intolerance gene elimination and gene inactivation during maturation of B-lymphocytes. Finally, the other problem that needs to be considered is the existence of non-functional genes. These genes refer to Ab genes containing stop codon(s) in their segments, either naturally or created by the vague rearrangement processes during the somatic recombination.

SUMMARY OF THE INVENTION

To overcome the drawbacks in the prior art, accordingly, one aspect of the invention provides a method for preparing a clone for producing an antigen-specific antibody, which comprises the steps of:

-   -   a) extracting genomic DNAs of lymphocytes from a mammalian         immunized with a specific antigen;     -   b) recovering variable regions of immunoglobulin heavy and light         chains from the genomic DNAs;     -   c) amplifying the variable regions using a frame-shifting PCR;     -   d) introducing both an adaptor and a linker to the variable         regions obtained in step c);     -   e) linking the variable regions of immunoglobulin heavy and         light chains obtained in step d) by an overlap-extensive PCR to         obtain single-chain variable fragments; and     -   f) introducing the single-chain variable fragments into a host         cell to form a clone.

Another aspect of the invention is to provide a method for preparing an antigen-specific antibody, comprising the step of expressing a clone prepared define herein.

Still another aspect of the present invention pertains to a method for retrieving either V_(L-κ) or V_(H) genes from a germline DNA comprising the steps of:

-   -   a) extracting genomic DNAs of lymphocytes from a mammalian         immunized with a specific antigen or from a non-immunized         mammalian;     -   b) recovering variable regions of immunoglobulin heavy and light         chains from the genomic DNAs; and     -   c) amplifying the variable regions using a frame-shifting PCR to         produce V_(L-κ) or V_(H) genes of variable lengths or sequences.

Yet another aspect of the present invention provides a method for constructing a library of the V_(L-κ) or V_(H) genes from a gemomic DNA comprising the steps of:

-   -   a) extracting lymphocytic genomic DNAs from a mammalian         immunized with a specific antigen or from a non-immunized         mammalian;     -   b) recovering variable regions of immunoglobulin heavy and light         chains from the genomic DNAs using a semi-nested PCR;     -   c) amplifying the variable regions using a frame-shifting PCR to         produce V_(L-κ) or V_(H) genes of variable lengths or sequences;         and     -   d) cloning the V_(L-κ) or V_(H) genes into a vector.

The procedure of extracting lymphocytic genomic DNAs from a mammalian is kwon well to those skilled in the art. The mammalian may be mice, rabbits, dogs and human being. In a preferred embodiment of the invention, the mammalian is a mouse, and the lymphocytes are lymphocytic CD19⁺ cells.

Preferably, the vector used in the invention is a TOPO TA vector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematic diagram of a frame-shifting PCR.

FIG. 2 shows a semi-nested and frame-shifting PCR amplification of Ig.

FIGS. 3A and 3B show cladogram of sequencing analysis of kapa-light (3A) and heavy (3B) chain variable region of Ig genes from non-immunized mouse splenocytic genomic DNA. The difference between the most related gene sequence is 23.6% and 62.3% for V_(L-κ) and V_(H), respectively.

FIG. 4 shows frame-shifting PCR verification: nucleotide sequence alignment of κ-light chain variable region of Ig genes derived from non-immunized mouse splenocytic genomic DNA. Sequences are frame-shifted V_(L-κ) genes which are subdivided into 7 families as shown in FIG. 3A, suggesting diversity-enhancement within the CDR3 region.

FIG. 5 shows phageELISA of candidate ScFv clones obtained after five rounds of panning against phOx-BSA conjugate (partly). ScFv clones with phageELISA reading more than 1.5 fold of the mean value of the sample set are isolated and subjected to further analysis. ScFv clones that are used for competitive phageELISA (FIG. 7) are marked with asterisks.

FIGS. 6A, 6B and 6C shows amino acid sequence alignment of heavy chain and light chain variable regions of isolated phox-spcific ScFv. Alignment result of different ScFv clones indicates subdivision into different families and significant sequence variations within the CDR3 region.

FIGS. 7A and 7B show result of phageELISA and competitive phageELISA, in which FIG. 7A shows dose response of candidate clones and FIG. 7B shows normalized competitive phageELISA of phOx-specific clones with the use of phOx as free ligand.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the present invention provides a method for preparing an antigen-specific antibody by constructing a library of a phage-displayed single chain variable fragment (ScFv) of an antibody using a novel frame-shifting PCR.

In the procedure of the frame-shifting PCR, diverse Ig genes (V_(L-κ) or V_(H) genes) of variable lengths and sequences are generated within the CDR3 region, so that defective Ig genes resulted from non-productive exon joining are recovered to hereby enhance the diversity in the CDR3 region. As shown in FIG. 1, Ig genes variable regions derived by a semi-nested PCR were served as template for the frame-shifting PCR which allows reverse primers sliding along the CDR3 region of the Ig gene variable regions. After the frame-shifting PCR modification, a library of the Ig variable regions with different lengths and sequences within the CDR3 region has been generated.

The semi-nested PCR and the frame-shifting PCR mentioned above are performed using a set of degenerated primers which cover most of Ig genes. According to the invention, a PCR primer (or, an oligonucleotide primer) is an oligonucleotide capable of specific hybridization under particular PCR conditions to a region of the template DNA. Each primer is typically used as a member of a primer pair, including a 5′ upstream primer that hybridizes with the 5′ end of the template DNA to be amplified and a 3′ downstream primer that hybridizes with the complement of the 3′ end of the template DNA to be amplified. Those skilled in the art to which the invention relates will understand primers useful in connection with the present invention may be prepared by any conventional DNA synthesis methods.

In the method for constructing a library of the V_(L-κ) or V_(H) genes, the V_(L-κ) or V_(H) gene fragments retrieved are cloned into a vector to construct the library. The vector used in the invention can be any vectors that conveniently express the genes in a host cell, including, but not limited to, bacterial plasmid vectors selected from expression, cloning, cosmid and transformation vectors such as pBR322, and animal viral vectors such as modified adenovirus, influenza virus, adeno-associated virus, polio virus, pox virus, retrovirus, and the like. Detecting and screening the library are performed by any of technology known to ordinary skill persons in the art.

In the method for constructing a mammalian single-chain variable region (ScFv) phage-displayed library, both an adaptor and a linker are introduced to the frame-shifted immunoglobulin gene variable regions. Then the heavy and κ/λ-light chains (V_(H) & V_(L-κ)) are linked so that the single-chain variable region (ScFv) is constructed. In one embodiment of the invention, the above procedure is carried out by two PCRs. That is the first PCR is an ordinary one and the second PCR is an overlap-extensive PCR. The resulted products are then cloned into a vector to constitute the library.

The present invention breaks through the limitation that using mRNA as an enriched source of expressed and spliced antibody genes neglects the allelic exclusive genes so as half of the potential gene by retrieving Ig genes from a germline DNA. A supplementary PCR strategy (“frame-shifting PCR”) introduces diversity into the CDR3 region of immunoglobulin so as to recover defective Ig genes resulted from non-productive exon joining and further to enhance the diversity in the CDR3 region. The method provided by the present invention for preparing antigen-specific antibody has been evaluated by construction of a small ScFv phage display library. Furthermore, ELISA and competitive phageELISA indicated significant differences in affinity among different clones.

A fast and simple method for retrieving the variable region of Ig genes and simultaneously introducing sequence diversity in the CDR3 region of antigen recognition domain is provided by the present invention.

It is understood that the antigen-specific monoclonal antibody prepared by the method of the present invention can be applied to prevent, diagnose and treat diseases of mammalians related to infection of the specific antigen, and to a kit containing the antibody.

The present invention will be further described with the following examples in conjunction with the drawings.

EXAMPLES Example 1 Construction of Libraries of V_(L-κ) or V_(H) Genes

Primers Used in the Examples

Primers used for amplification reactions were listed as follows: A. Mouse V _(H)  Forward Primers (FR1 Region): F1: 5′- gAggTgMWgcTKVWg F2: 5′- gAggTgMWgcTKVWgSAgTcTggA SBS1: 5′- cgAgcTcggATccggcccAgccggccSAggTgMWgcTKV WgSAg B. Mouse V _(H)  Reverse Primers (FR4 Region): R1: 5′- gAcDgTgASHRDRgT R2: 5′- gAcDgTgASHRRDRgTBccTKSRccccA R3: 5′- gAcDgTgASHRDRgTBccTKSRccccANNNNNN L1JP: 5′- AgAAccgcTgccTgAAccgccTccAccAcTgAcDgTgASHR DRgTBccT C. Mouse V _(L-κ)  Forward Primers (FR1 Region): F11: 5′- gAHRTYgTKMTSAc F12: 5′- gAHRTYgTKMTSAcMcARWcTMcA L2TP: 5′- TcAggcAgcggTTcTAgcggcggTggcggAgAHRTYgTKM TSAcMcARWc D. Mouse V _(L -κ)  Reverse Primers (FR4 Region): R11: 5′- KATYTccARYYTKgT R12: 5′- KATYTccARYYTKgTSccHBcDccgAA R13: 5′- YYTKgTSccHBcDccgAAYgTNNNNNN KN1: 5′- cggggTAccgcggccgcKATYTccARYYTKgTSccHBc DccgAA E. Mouse V _(L-λ)  Forward Primers (FR1 Region): F21: 5′- cAggcTgTTgTgA F22: 5′- cAggcTgTTgTgAcTcAggAATcT L3JP: 5′- TcAggcAgcggTTcTAgcggcggTggcggAcAggcTgTT gTgAcTc F. Mouse V _(L-λ)  Reverse Primers (FR4 Region): R21: 5′- AccTAggAcAgTcA R22: 5′- AccTAggAcAgTcAVYYTggTTcc R23: 5′- AgTcAVYYTggTTccWcTNcMgAAMAYNNNNNN KN2: 5′- cgggTAccgcggccgcAgTcAVYYTggTTccWcYNcMgAA In the above, the non-standard bases are:

-   U=deoxyuracil; I=deoxyinosine; N=A+c+T +g; R=A+g; Y=c+T -   M=A+c; K=T+g; S=c+g; W=A+T; H=A+T+c; B=T+c+g -   D=A+T+g; V=A+c+g     Amiplifying Genomic DNAs Extracted from Lymphocytes

The CD19⁺ lymphocytes were isolated from spleen of a non-immunized Balb/C mouse. Genomic DNA was extract by use of DNAzol Reagent (Cat #: 10503-027) from GibcoBRL Life Technology with procedures as detailed by the manufacturer. Particularly, 1 ml of DNAzol reagent was added to splenocytic lymphocytes (10⁷) and cells were lysed by gently pipetting up and down. The cell lysate was centrifugated for 10 minutes at 10,000 g at 4° C. After centrifugation, the viscous supernatant was transferred to a new tube. DNA was precipitated by adding 0.5 ml of 100% ethanol per ml of DNAzol used. The sample was mixed by inversion and then incubated at room temperature for 3 minutes. The cloudy DNA precipitate was removed from tube by spooling with a pipette tip and then transferred to a new tube. After washing twice with 1 ml of 75% ethanol, the DNA precipitate was air-dried and then re-dissolved in distilled water for next used.

A mixture containing the following components was used to amplify the Ig gene variable region from the genomic DNA. V_(H) V_(L-κ) Final Name Vol. (μl) Vol. (μl) Conc. 1) 10X PCR buffer 5 5 1X 2) 25 mM MgCl₂ 3 3  1.5 mM 3) 10 mM dNTP 1 1  0.2 mM 4) 10 μM F1 V_(H) primer 1.5 —  0.3 μM 5) 10 μM R1 V_(H) primer 6.5 —  1.3 μM 6) 10 μM F11 V_(L-k) primer — 2.25 0.45 μM 7) 10 μM R11 V_(L-k) primer — 1.5  0.3 μM 8) 100% DMSO 2.5 2.5 5% 9) Splenocytic genomic DNA x x 100 ng/50 μl 10) 5 U/μl Taq. Polymerase 0.5 0.5 0.05 U/μl 11) Sterilized H₂O x x make up to 50 μl Total 50 50 (μl) PCR condition: 30 cycles (˜1.5 hr)

For both V_(H) and V_(L-κ): Post- Preheating Denaturation Annealing Extension extension Storage 94° C. 94° C. 40° C. — — 4° C. 45 sec. 15 sec. 1.5 min. — — infinity

Products of the amplification reaction were analyzed by agarose gel electrophoresis. The result was shown in FIG. 2. The PCR product of genomic DNA amplification with primer pair for heavy chain of Ig genes was shown in lane 1.

Recovering Variable Regions of Ig Heavy and κ-Light Chains by Semi-Nested PCR

The PCR product of the first round PCR amplification of V_(H) and V_(L-κ) were used as templates for the 2nd round semi-nested PCR of Ig heavy and κ-light chains, respectively. A mixture used in the amplification included: V_(H) V_(L-κ) Final Name Vol. (μl) Vol. (μl) Conc. 1) 10X PCR buffer 5 5 1X 2) 25 mM MgCl₂ 3 3  1.5 mM 3) 10 mM dNTP 1 1  0.2 mM 4) 10 μM F2 V_(H) primer 1.5 —  0.3 μM 5) 10 μM R2 V_(H) primer 7.5 —  1.5 μM 6) 10 μM F12 V_(Lk) primer — 2.5  0.5 μM 7) 10 μM R12 V_(Lk) primer — 5.6 1.12 μM 8) 100% DMSO 2.5 2.5 5% 9) 1^(st) PCR mixture, (V_(H)/V_(Lk)) 3 3 (μl) 10) 5 U/μl Taq. Polymerase 0.5 0.5 0.05 U/μl 11) Sterilized H₂O 26 26.9 make up to 50 μl Total 50 50 (μl) PCR condition: 35 cycles and

For V_(H): Post- Preheating Denaturation Annealing Extension extension Storage 94° C. 94° C. 65° C. — 65° C. 4° C. 2 min. 30 sec. 40 sec. — 2 min. infinity

For V_(L-κ): Post- Preheating Denaturation Annealing Extension extension Storage 94° C. 94° C. 58° C. — 58° C. 4° C. 2 min. 30 sec. 40 sec. — 2 min. infinity

Variable regions of immunoglobulin heavy and κ/λ-light chains (V_(H) & V_(L-κ)) were obtained by the semi-nested PCR amplification. The PCR products were run on agarose gel with ethidium bromide staining and ultraviolet-light visualization. The result was shown in lanes 2 and 3 of FIG. 2. Following electrophoretic migration, the gel was removed and the amplified products were extracted from the gel.

Amplifying Immunoglobulin Gene Variable Regions by Frame-Shifting PCR

A frame-shifting PCR was carried out using templates of V_(H) & V_(L-κ) produced by the semi-nested PCR. A mixture used for the frame-shifting PCR contained the following components: V_(H) V_(L-κ) Final Name Vol. (μl) Vol. (μl) Conc. 1) 10X PCR buffer 5 5 1X 2) 25 mM MgCl₂ 3 3 1.5 mM 3) 10 mM dNTP 1 1 0.2 mM 4) 10 μM F1* V_(H) primer 2.5 — 0.5 μM 5) 10 μM R3 V_(H) primer 20 —   4 μM 6) 10 μM F12 V_(Lk) primer — 2.5 0.5 μM 7) 10 μM R13 V_(Lk) primer — 20   4 μM 8) Gel-extracted semi-nested x x 60 ng/50 μl PCR pdt. (snV_(H)/snV_(Lk)) 9) 5 U/μl Taq. Polymerase 0.5 0.5 0.05 U/μl 10) Sterilized H₂O x x make up to 50 μl Total 50 50 (μl) PCR condition: 25 cycles (Overnight: ˜8 hrs) and

For both V_(H) and V_(L-κ): Post- Preheating Denaturation Annealing Extension extension Storage 94° C. 94° C. 20° C. — 4° C. 2 min. 30 sec. 2 min. ramping — infinity up w/speed 0.1° C./sec.

The amplified products were analyzed by agarose gel electrophoresis (FIG. 2, lane 4). In the procedure of the frame-shifting PCR, diverse Ig genes (V_(L-κ), or V_(H) genes) of variable lengths and sequences within the CDR3 region were generated. As a result, diversity was introduced into the CDR3 region of the immunoglobulin genes so that defective Ig genes resulted from non-productive exon joining were recovered to hereby enhance the diversity in the CDR3 region as shown in FIG. 1.

Transformation and Sequencing Analysis

Libraries of V_(H) and V_(L-κ) variable region derived from the frame-shifting PCR were cloned into TOPO TA vector. Particularly, ligation mixture was used to transform TG1 or DH5α E. coli cells either by standard calcium chloride-mediated transformation, or by electroporation at 1900 V/cm, resistance at 200Ω, and the capacitance at 50 μF with 0.1 cm electroporation cuvettes and a Bio-Rad electroporator. Transformed competent cells were plated on agar plates containing ampicillin (100 μg/ml) and incubated at 37° C. overnight. Transformed colonies were randomly picked from the agar plates. Plasmid DNA was prepared from overnight bacterial culture by use of QIAprep Spin Miniprep kit (Qiagen). Purified plasmid were used for sequence determination of V_(H) and V_(L-κ) clones by fluorescent dye-labelled terminator cycle sequencing with a Beckman CEQ 2000 autosequencer.

120 transformants from each library were randomly picked for sequencing analysis. Phylogenic analysis indicated that, with respect to the germline Ig sequences in NCBI database as reference sequences, the 31 and 33 full-length sequenced V_(L-κ) or V_(H) clones can be classified into 7 and 8 different families, respectively (FIG. 3). Moreover, multiple sequence aligmnents revealed that significant sequence differences within the CDR3 region among those sequenced clones (FIG. 4).

In this experiment, we aimed at providing proof-of-concept evidence for frame-shifting PCR. Specifically two questions were addressed: (1) non-immunised genomic DNA can be used as template for construction of immunoglobulin chain library by frame-shifting PCR; (2) application of frame-shifting PCR would generate an immunoglobulin chain-library consisting of heterogeneous Ig sequences with significant diversity in the CDR3 region (FIG. 1). As illustrated in FIG. 2, variable region of immunoglobulin chains in the genomic DNA can be efficiently recovered by the semi-nest PCR steps though present in minutes amount (FIG. 2, lane 1-lane 2). With the use of gel-purified ˜340-bp PCR product of semi-nest PCR (FIG. 2, lane 3) as a template, frame-shifting PCR generates a library of DNA segments that is heterogeneous in size (FIG. 2, lane 4). Following TA-cloning, the nucleotide sequence of frame-shifting PCR products was determined. To analyze the diversity of the immunoglobulin chain libraries, full-length sequences of randomly picked clones of a library were subjected to phylogeny analysis with a web-based software (ClustalW of European Bioinformatics Institute). As indicated in FIG. 3, clones can be grouped into different families, suggesting the library consisting of heterogeneous immunoglobulin chains. Furthermore, to evaluate whether frame-shifting PCR enhances CDR3 diversity as predicted (FIG. 1), clones of the V_(L-κ) library were subjected to multiple sequences alignment. As shown in FIG. 4, clones of each family are mostly identical except notable sequence difference in the CDR3 region. The result suggests that clones of each family might be derived from a single template, and diversity is introduced into the CDR3 region by frame-shifting PCR.

Example 2 Construction of Single Chain Variable Antibody Fragment (scFv)

Construction of Libraries of V_(Lκ) and V_(H) Genes

Libraries of V_(L-κ) and V_(H) Genes were constructed as described in Example 1, except the CD19⁺ lymphocytes were isolated from spleen of a Balb/C mouse that was immunised with 4-ethoxymethylene-2-phenyl-2-oxazolin-5-one (phOx) conjugated to chicken serum albumin (CSA, Sigma, USA).

Construction of Single Chain Variable Antibody Fragment (ScFv)

Both an adaptor and a linker were introduced into the frame-shifted immunoglobulin gene variable regions by PCR using a reactive mixture including the following components: V_(H) V_(L-κ) Final Name Vol. (μl) Vol. (μl) Conc. 1) 10X PCR buffer 5 5 1X 2) 25 mM MgCl₂ 3 3 1.5 mM 3) 10 mM dNTP 1 1 0.2 mM 4) 10 μM SBS1 V_(H) adaptor 1 — 0.2 μM 5) 10 μM L1JP V_(H) Linker 1 — 0.2 μM 6) 10 μM L2JP V_(Lk) Linker — 1 0.2 μM 7) 10 μM KN1 V_(Lk) adaptor — 1 0.2 μM 8) 100% DMSO 2.5 2.5 5% 9) Gel-extracted x x 60 ng/50 μl frame-shifted PCR pdt. (fsV_(H) or fsV_(Lk)) 10) 5 U/μl Taq. Polymerase 0.5 0.5 0.05 U/μl 11) Sterilized H₂O x x make up to 50 μl Total 50 50 (μl)

PCR condition: 15 cycles (˜1 hr) and for both V_(H) and V_(L-κ): Post- Preheating Denaturation Annealing Extension extension Storage 94° C. 94° C. 54° C. 72° C. 72° C. 4° C. 2 min. 30 sec. 30 sec. 30 sec. 2 min. infinity

A resultant product followed an overlap-extensive PCR to construct single-chain variable regions in which the V_(H) gene with an adaptor and a linker was connected to the V_(L-κ) gene with a linker and an adaptor. A mixture used for overlap-extensive PCR comprised the following components. ScFv Final Name Vol. (μl) Conc. 1) 10X PCR buffer 5 1X 2) 25 mM Mg₂Cl 3 1.5 mM 3) 10 mM dNTP 1 0.2 mM 4) 10 μM SBS1 V_(H) adaptor 1 0.2 μM 5) 10 μM KN1 V_(Lk) adaptor 1 0.2 μM 6) 100% DMSO 2.5 5% 7) Adaptor linked fsV_(H) 5 (PCR product) 8) Adaptor linked fsV_(Lk) 5 (PCR product) 9) 5 U/μl Taq. Polymerase 0.5 0.05 U/μl 10) Sterilized H₂O 26 make up to 50 μl Total 50 (μl)

PCR condition: 20 cycles (˜1 hr) and Post- Preheating Denaturation Annealing Extension extension Storage 94° C. 94° C. 58° C. 72° C. 72° C. 4° C. 2 min. 30 sec. 30 sec. 30 sec. 2 min. infinity Transformation, Identification and Analysis

The single-chain variable regions produced were separated by agarose gel electrophoresis and potential candidates cloned into the pCANTAB 5E phagemid vector (Amersham Pharmacia Biotech Inc.) and rescued phage clones were selected against EM-phOx (purchased from Sigma, USA) with standard biopanning procedures. Particularly, following transformed into E. Coli TG1 cells, phage clones displaying the ScFv antibody as surface ScFv-g3 fusion proteins were rescued from the transformed TG1 cells with M13KO7 helper phage. To select phage clones that are selectively bound to phOx, rescued phage clones were incubated in a 24-well tissue culture plate (0.5 ml/well) that was coated with phOx conjugated to bovine serum albumin (BSA) for 2 hours at room temperature. After removing unbound phage by washing 5 times with 2 ml/well of phosphate-buffered saline (PBS) and then 10 times with 2.5 ml/well of PBS containing 0.1% Tween-20. Bound phage clones were eluted by adding 100 ul of 0.1M glycine-HCl (pH=2.2) and incubated for 10 minutes at room temperature. Eluted clones were collected, neutralized by adding 10 ul of 1M Tris-HCl (pH=8.0). Pooled phage clones were then used to re-infect TG1 cells. Enriched phage clones were then rescued from transformed TG1 cells with M13KO7 helper phage. The biopanning cycle was repeated 5 times, and then individual phage clones were isolated.

After 5 rounds of panning against phOx conjugated to bovine serum albumin (BSA), potential candidate clones (9.7×10⁵ recombinants) were identified. 288 Clones were randomly picked and their reactivities against phOx were determined by phageELISA. Forty-four highly reactive clones, of which reactivity towards EM-phOx was 1.5-fold higher than the mean value of the sample set, were isolated and further analyzed (FIG. 5). Phylogenic analysis of the sequences suggested that the derived Ig genes could be grouped into different classes and significant sequence variations were found within the CDR3 region of Ig genes in each class (FIG. 6). Furthermore, with the use of phOx as free ligand, competitive phageELISA indicated significant differences in affinity among different clones (FIG. 7).

In this experiment, we aimed at testing the feasibility of using frame-shifting PCR to prepare target specific scFv antibodies. Specifically, with the use of a small molecule phenyloxazolone as a model antigen, we have prepared an scFv library by using genomic DNA that derived from CD19⁺ lymphocytes of an immunized Balb/C mouse as template for frame-shifting PCR. For screening scFv antibodies that selectively reacted with phOx, we expressed the scFv library as a phage-display scFv library, and then biopanning against immobilized phOx. Isolated individual phage clone was then used as a primary antibody for testing its reactivity towards phOx by an ELISA (phageELISA). A typical result of the phageELISA assay was shown in FIG. 5, and phage clones that gave high reactivity towards phOx were then further subjected for nucleotide sequence determination. Alignment of the sequences showed that the isolated phOx-specific scFv clones displayed significant sequence variation in the CDR3 region (FIG. 6), suggesting that frame-shifting PCR is capable to generate and to retrieve heterogeneous functional scFv antibodies. On the other hand, it is predicted that a highly diverse antibody library should consist of scFv clones displaying a wide range of affinity toward antigens. Hence we compared the affinity of the isolated phOx-specific scFv clones by saturation analysis with phageELISA (FIG. 7A), and by a competitive phageELISA in which free phOx was used to compete with immobilized phOx for binding to the isolated scFv clones (FIG. 7B). Indeed, isolated phOx-specific phage clones bound to immobilized phOx in a concentration-dependent manner, with affinity (EC50) approximately spanning one and a half log scale amongst different scFv clones (FIG. 7A). In agreement with saturation analysis, binding of scFv towards immobilized phOx were concentration-dependently displayed by free phOx, and the IC50 value of various scFv clones approximately spanning through two log scale (FIG. 7B). These results imply that frame-shifting PCR produces highly diverse scFv library consisting of target-specific scFv antibodies characterized with a wide spectrum of affinities. 

1. A method for preparing a single-chain variable fragments coding an antigen-specific antibody, comprising the steps of: a) extracting genomic DNAs of lymphocytes from a mammalian immunized with a specific antigen; b) recovering variable regions of immunoglobulin heavy and light chains from the genomic DNAs; c) amplifying the variable regions using a frame-shifting PCR; d) introducing both an adaptor and a linker to the variable regions obtained in step c); and e) linking the variable regions obtained in step d) by an overlap-extensive PCR to obtain single-chain variable fragments.
 2. The method of claim 1, wherein the step b) is performed by a semi-nested PCR.
 3. The method of claim 2, wherein the mammalian is a mouse.
 4. The method of claim 2, wherein the specific antigen is 4-ethoxymethylene-2-phenyl-2-oxazolin-5-one conjugated to chicken serum albumin.
 5. The method of claim 1, wherein the frame-shifting PCR comprises primers designed for the variable regions, which cover most of the immunoglobulin genes.
 6. The method of claim 5, wherein the primers are gAggTgMWgcTKVWg and gAcDgTgASHRDRgTBccTKSRccccANNNNNN for V_(H).
 7. The method of claim 5, wherein the primers are gAHRTYgTKMTSAcMcARWcTMcA and YYTKgTSccHBcDccgAAYgTNNNNNN for V_(L-κ).
 8. The method of claim 5, wherein the frame-shifting PCR for both V_(H) and V_(L-κ) is performed with a condition as follows: preheating at 94° C. for 2 min, then followed with 25 cycles of denaturation at 94° C. for 30sec, annealing at 20° C. for 2 min; extension at ramping up w/speed 0.1° C./sec, and then storage at 4° C.
 9. The method of claim 1, wherein the lymphocytes are splenoctic CD⁺19 cells.
 10. A method for preparing an antigen-specific antibody comprising: a) extracting lymphocytic genomic DNAs from a mammalian immunized with a specific antigen; b) recovering variable regions of immunoglobulin heavy and light chains from the genomic DNAs; c) amplifying the variable regions using a frame-shifting PCR; d) introducing both an adaptor and a linker to the variable regions obtained in step c); e) linking the variable regions obtained in step d) by an overlap-extensive PCR to construct single-chain variable fragments; and f) introducing the single-chain variable fragments into a host cell and expressing the cell.
 11. The method of claim 10, wherein the antigen-specific antibody is a monoclonal antibody.
 12. The method of claim 11, wherein the monoclonal antibody is a monoclonal single-chain variable fragments.
 13. A method for retrieving either V_(L-κ) or V_(H) genes from a genomic DNA comprising the steps of: a) extracting lymphocytic genomic DNAs from a mammalian; b) recovering variable regions of immunoglobulin heavy and light chains from the genomic DNAs using a semi-nested PCR; and c) amplifying the variable regions using a frame-shifting PCR to produce V_(L-κ) or V_(H) genes of variable lengths or sequences.
 14. The method of claim 13, wherein the mammalian is immunized with a specific antigen.
 15. The method of claim 14, wherein the mammalian is mouse and the specific antigen is 4-ethoxymethylene-2-phenyl-2-oxazolin-5-one conjugated to chicken serum albumin.
 16. A method for constructing a library of V_(L-κ) or V_(H) genes from a genomic DNA comprising the steps of: a) extracting lymphocytic genomic DNAs from a mammalian; b) recovering variable regions of immunoglobulin heavy and light chains from the genomic DNAs using a semi-nested PCR; c) amplifying the variable regions using a frame-shifting PCR to produce V_(L-κ) or V_(H) genes of variable lengths or sequences; and d) cloning the V_(L-κ) or V_(H) genes into a vector.
 17. The method of claim 16, wherein the mammalian is immunized with a specific antigen.
 18. The method of claim 17, wherein the mammalian is mouse and the specific antigen is 4-ethoxymethylene-2-phenyl-2-oxazolin-5-one conjugated to chicken serum albumin.
 19. The method of claim 16, wherein the vector is a TOPO TA vector.
 20. The method of claim 18, wherein the vector is a TOPO TA vector. 